Another aspect of prop drag

[Guest]

For years I have understood that a prop locked in a static position when sailing produces less drag than the same prop being allowed to freewheel. Recently I have come across a couple of very positive statements the a freewheeling prop produces less drag than a locked prop.

Does anyone knoew if real comparative tests have ever been carried out and the results published?

 

[Guest]

Talk to a helicopter pilot

The drag will the proportion of to the resistance of the shaft and the inefficiency of the propeller.
For I'm not sure about where you would find researched results, but I would try for the amateur yacht research people or the US Navy (most of their research is open access) closer to home and Southampton University or Southampton Institute may give you a pointer. Alternatively you could try the wind generating people



-{)>

 

[Mirelle]

Re: Talk to a helicopter pilot

Most yacht gearboxes cannot cope with a freewheeling propeller, so for most boats the question is a purely academic one. However, I am pretty sure that way back in the 1960's when Douglas Phillips-Birt wrote the "Reflections in the Sea" column in YM under the name of "Argus" he touched on this, citing some towing tank work. Alas, I don't recall the conclusion....

 

[PeterGibbs]

Well, this is an old chestnut.

For myself I have done the old grey whistle test on a number of occasions. IE with and without locked prop, and on a straight prop shaft and, now, an S leg drive (Volvo) On each occasion I detected a shift on the log favouring a loose running prop (3 bladed in both cases) I found it was worth about 1/3 knot to keep the propmoving - which is what I always do now.

Some protest that the rumbling sound some gearboxes make is not worth the hassle - well, that's a question of taste.

On a theoretcical level I could never see the argument for a fixed (prop) drag being less negative than a yielding(rotating) drag - ie an on obstruction actually moving at least partly in sympathy with the flow.

 

[ranga]

In the late 80s I sailed on one of 2 Catalina 36s across the Atlantic in convoy. They were identical brand new boats and we found that with the prop freewheeling, the boat was slightly faster. It seemed a pretty good "tank test" over a few thousand miles as we could judge the difference in speed by the distance increasing or decreasing between the 2 boats whenever we either locked it in place or let it run free.

 

[ccscott49]

If you have borg warner velvet drive boxes, the oilpump is on the input shaft, if the prop is left freewheeling for long periods, the bearings get starved of oil, causing uneccessary wear, the makers say its OK, But! and from good reports all over the place, damage does occur, you might take this into consideration with the prop rotating or not scenario. I now lock mine after having found out about the bearing wear and have seen no change in performance, but she is a motor sailer. Also on some stern tube arrangements the inner bearing is fed with water from the engine for cooling and lubrication, if the prop is left turning without the engine on, the shaft is starved of lubrication, I have those type stern glands, which is also a rteason to lock the prop!

 

[gunnarsilins]

I did try this out.....

....on my Moody 42 ketch with an 24" 3-bladed prop and a shaft lock. I had my electronic log and I spent pretty much time under sail in various wind speeds releasing the shaft brake and tightening it again.

When freewheeling the prop slowed the boat down, depending on the wind force and boatspeed.
In very slow speeds (< 2 kn) there weas no difference because the waterflow could not override the friction. But as soon the speed went higher and the prop begun to rotate I lost 0,25 knots (!) if not locking it.
The higher the speed , the less was the difference between locked and unlocked (>7 knots, no difference).

I think, in theory would the free wheeling prop cause less drag but when you consider the friction in sterngear and gearbox the situation will be the other way round. Normally, turnung the shaft by hand shows pretty much stiffness....

At least on my yacht locking the shaft was the best alternative, also as a bonus, less sound and less wear.

But finally I went for a Maxprop feathering instead, and now I talk BIG difference!